In a modern densitometer, the light source usually consists of an incandescent lamp supplied with a constant voltage. The lamp is switched on immediately before each measurement and switched off immediately after the measurement. This intermittent lamp operation is for various reasons, including the saving of energy in battery-operated devices and for reducing the wear and increasing the life of the lamp. It has been observed that in the case of intermittent operation the luminous flux from the lamp does not remain constant in time in the period immediately following switch on. Immediately after the constant voltage has been switched on, the flux rises a few percent above the final desired value and then slowly falls to the constant set value during a relatively long time, up to a second or more. This overshooting of the luminous flux results in errors in linearity when photo-electrically measuring the object under test, and these errors cannot be compensated by the measuring circuit itself. The reason is that if, as is common today, the measurement process includes an integration method with variable integration period, the overshooting luminous flux affects the result to a varying extent, depending on the integration period.
Theoretically, of course, the aforementioned error in linearity could be eliminated by adjusting the lamp voltage to obtain a constant luminous flux from the moment of switch on. However, such an adjustment would require an additional photodetector and a complicated regulating circuit. This is undesirable in terms of cost and often impossible for lack of space or other reasons.
Another possibility of reducing or eliminating the overshoot of the luminous flux would of course be to switch the lamp on permanently or permanently preheat it at a reduced voltage. As already mentioned, however, this method is disadvantageous with regard to energy consumption and the life of the lamp.
The protection of an incandescent lamp to extend its life in a measuring instrument is described in German Offenlegungschrift No. 2 038 603 which relates to a reflection photometer and discloses a lamp which is protected by being operated at a considerably reduced voltage. A colorimeter type measuring instrument having an intermittently operated lamp is disclosed in U.S. Pat. No. 3,653,764 to Schodl. In this case the light colour as well as intensity is of importance. The lamp is protected at each switching on by being operated initially at constant current and then at constant voltage. A blanking gate system is provided to ensure the measuring signal obtained with the aid of the lamp is transmitted only during the latter portion of the switch on period when the light flux is essentially constant. Such measures are relatively complex.
It is also known to preserve incandescent lamps for airport lighting systems by having a control system for "gently" switching them on. Such a system is disclosed in French laid-open patent specification No. 2 414 277. The disclosed control system provides on switch on a continuous increase in voltage to the lamps rising slowly from zero to the rated value. As can be seen from FIG. 2 of the French specification, the period of the rising voltage is long and is too long for practical use in an instrument such as a densitometer.
It is, therefore, an object of the present invention to provide a means for controlling the energisation of an intermittently operable incandescent lamp in a densitometer or like measuring instrument whereby overshoot of the luminous flux above the set level is avoided on switching on, and further to provide such control means in a relatively simple and inexpensive manner.